1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery and a method of manufacturing the same. Specifically, the invention relates to a separator for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery which is manufactured using the separator.
2. Description of Related Art
In a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery, additives are added to a nonaqueous electrolyte, for example, for one or two or more of the purposes including: improvement of the storability of the battery; improvement of cycle characteristics; improvement of input and output characteristics; and improvement of initial charge-discharge efficiency. For example, Japanese Patent Application Publication No. 2014-035955 (JP 2014-035955 A) describes that low-temperature discharge characteristics of a battery can be improved by adding a film forming agent (specifically, lithium difluorophosphate (LiPO2F2)) to a nonaqueous electrolyte.
Typically, the film forming agent added to the nonaqueous electrolyte is electrically decomposed (oxidation decomposition or reduction decomposition) on a positive electrode and/or a negative electrode to produce a protective film containing a decomposition product of the film forming agent on a surface of the electrode. As a result, an interface between the electrode (typically, an active material) and the nonaqueous electrolyte can be maintained in a stable state.
However, according to an investigation by the present inventors, it was found that there is room for improvement in the above-described technique. That is, in the battery disclosed in JP 2014-035955 A in which lithium difluorophosphate is added to the nonaqueous electrolyte, most (typically, 60% or more, for example, 80% or more with respect to the total addition amount) of the added lithium difluorophosphate is reduced and decomposed on the negative electrode and is deposited on the surface of the negative electrode as a film. Therefore, an excess amount of the film is formed on the negative electrode. Accordingly, the battery resistance may increase, and the intercalation of lithium ions may not follow the formation of the film. As a result, lithium metal is deposited on the surface of the negative electrode, which may decrease the input and output characteristics or durability (for example, Li deposition resistance) of the battery. On the other hand, since most of the added film forming agent is consumed on the negative electrode, the formation of the film on the positive electrode may be insufficient. As a result, for example, after the battery is exposed to a severe environment (for example, to a high-temperature environment of 50° C. or higher) for a long period of time, an interface between the positive electrode and the nonaqueous electrolyte may be unstable, and the battery resistance may increase.